Acid zinc plating baths

ABSTRACT

An acid zinc electroplating bath which contains a brightening amount of a soluble thiourea of the formula   D R A W I N G

United States Patent 11 1 Martin [4 1 Nov. 6, 1973 [5 AC) ZINC PLATINGBATHS 2,696,467 12 1954 Overcash et al. 204/52 R 2,742,412 4/1956Cransberg et al..... 204/52 R [751 lnvemor- Mam", Dem)", 2,910,413 101959 Strauss et al 204 44 73 Assignee; o Metai Finishing Corporation,3,502,551 3/1970 Todt et al. 204/52 R warren, Mich 3,558,448 1/1971Accarles et al. 204/55 R Flledi y 1971 Primary Examiner-F. C. Edmundson[2]] Appl No; 146,445 Attorney-Wm. J. Schramm et al.

[63] ggg sgg z gg of 822941 May An acid zinc electroplating bath whichcontains a brightening amount of a solublethiourea of the formula [52]11.8. C1. 204/55 R, 204/D1G. 2 s [51] Int. Cl C23b 5/12 ll [58] Field ofSearch 204/55, 55 R, 55 Y wherein R is independently selected from thegroups References Clled consisting of hydrogen, alkenyl of up to 4carbon atoms UNITED STATES PATENTS and alkynyl of up to 4 carbon atoms,provided that at 3,655,533 4 1972 Page et a1. 204 55 R least one R isalkenyl or y y The Zinc P g baths 2,080,479 5/1937 Hoff 204455 Y may beeither acid sulfate or acid fluoroborate baths 2,828,252 3/1958Fischer.... 204/55 Y and desirably contain the thiourea compound in an1,903,860 4/1933 Gockel 0 /55 Y amount of at least about 0.01 grams perliter. 2,563,360 8/1951 Phillips et al. 204/52 R 2 Claims, No DrawingsACID ZINC PLATING BATHS This application is a continuation-in-part ofU.S. Ser. No. 822,041 filed May 5, 1969, now abandoned.

This invention relates to improvements in the electrodeposition of zincand more particularly it relates to improved compositions and processesfor the electrodeposition of bright zinc from acid zinc plating baths.

Heretofore, a great deal of the bright zinc electroplating has beencarried out using alkaline zinc electroplating baths which containcyanide. Although such zinc plating baths have generally beensatisfactory from the standpoint of the characteristics of the brightzinc electroplate produced, because of the highly toxic nature of thecyanide constituents, the use of such baths has involved considerableproblems in the area of waste disposal. These problems have beenaccentuated recently by the adoption of more stringent laws andregulations on pollution;

Although acid zinc plating baths, instead of those containing cyanide,are not new, the use of such acid plating solutions does present someproblems and, hence, these have not been widely adopted. Among theproblems involved in using such acid zinc plating solutions is that itis often difficult to obtain a zinc plate having the desired brightness.Additionally, it has been found that even where the desired bright zincplate is obtained, such plate is often undesirably brittle.

It is, therefore, an object of the present invention to provide animproved acid zinc plating composition for the electrodeposition of abright zinc plate.

Another object of the present invention is to provide an improvedprocess for bright zinc plating which utilizes an acid zinc platingsolution.

A further object of the present invention is to provide an improved zincplate having a uniform, high degree of brightness, which plate is notundesirably brittle.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

Pursuant to the above objects, the present invention.

includes an electroplating bath which comprises an aqueous acidicsolution of an inorganic zinc salt, which solution contains abrightening amount of a soluble thiourea material of the formula over arelatively wide range of operating conditions.

More specifically, the plating baths of the present invention areaqueous acidic solutions containing zinc sulfate or zinc fluoroborate,and a brightening amount of one or more soluble thiourea materials. Acidzinc plating baths, whether based on zinc sulfate or on zincfluoroborate are conventional and known in the art. When based on zincsulfate, these baths typically contain zinc sulfate in an amount withinthe range of about 50 to 400 grams per liter. Additionally, such bathsmay also contain conductive salts to improve the bath conductivity suchas ammonium sulfate, ammonium chloride or bromide and the like, whichsalts are typically present in amounts within the range of about 10 to30 grams per liter. Additives to control the pH of the bath, such asboric acid, ammonium sulfate, and the like, may also be included,typically in amounts within the range of about 10 to 30 grams per liter.Additionally, additives such as aluminum sulfate, dextrin, licorice,glucose, or the like may also be added, typically in amounts within therange of about 1 to 30 grams per liter, to improve the crystal structureof the zinc plate obtained and provide a wider operating range ofcurrent density.

With plating baths based on zinc fluoroborate, the zinc fluoroborate istypically present in amounts within the range of about 50 to 300 gramsper liter. Such baths may also contain one or more ammonium salts, suchas ammonium chloride, ammonium fluoroborate, and the like, whichmaterials are typically present in amounts within the range of about 10to 30 grams per liter.

In addition to the above components, the electroplating baths of thepresent invention also include a brightening amount of at least onesoluble thiourea compound. By a soluble thiourea material, it isintended that the materials used have a sufficient solubility in theaqueous acidic plating baths so that a brightening amount of thethirouea material will be dissolved in the bath. In many instances, suchbrightening amount of the thiourea materials has been found to be atleast about 0.01 gram per liter. The maximum amount of the thioureabrightening agent may be up to the limit of its solubility in theaqueous acidic plating bath and will, thus, vary, depending upon theparticular thiourea material used. In many instances, it has been foundthat amounts of thiourea brightener in excess of about 10 grams perliter do not result in any appreciable increase in the brightness of theplate which is obtained. Preferably, the amount of the thioureabrightener in the acid plating bath of the present invention is withinthe range of about 0.01 to 10 grams per liter.

The soluble thiourea materials used desirably are of the formula:

wherein R is independently selected from the group consisting ofhydrogen, alkenyl of up to 4 carbon atoms and alkynyl of up to 4 carbonatoms, provided that at least one R is alkenyl or alkynyl.

It is to be appreciated, that various thiourea materials, as encompassedby the definition set forth hereinabove which are soluble in the platingbaths, may be used as brighteners in the electroplating baths of thepresent invention. Exemplary of specific thiouria materials which may beused are N-allylthiourea; N,N'- ethylenethiourea; N,N-dibutynylthiourea;N,N'- dibutenylthiourea and the like. It is to be appreciated, however,that the above are merely exemplary of the various thiourea materialswhich may be used as brighteners, and that other similar materials, asfall within the definition set forth hereinabove, may also be utilizedto obtain satisfactory results.

The electroplating baths of the present invention may be used over awide range of operating conditions. Desirably, the bright zinc plate iselectrodeposited from the bath using an average cathode current densityof from about 10 to amps per square foot, with bath temperatures withinthe range of about 15 to 40 C. If

desired, during plating the bath may be agitated with air ormechanically, or the work pieces may themselves be mechanicallyagitated, or plating may be carried out in a non-agitated bath.Preferably, during plating, the bath is maintained at a pH within therange of about 2.7 to 5.8, the desired pH generally being maintained bythe addition of a suitable material, such as ammonium hydroxide, zinccarbonate, or the parent acid of the zinc salt in the system.

By the use of the method of the present invention, there iselectrodeposited an excellent, bright zinc plate, which plate is notundesirably brittle. Moreover, it is found that this bright zinc plateis produced even in areas of low current density and that in highcurrent density areas, there is little or no evidence of burning of thezinc electroplate deposit. Additionally, it is found that since onlyrelatively small amounts of the thiourea material brighteners arerequired to produce these results, the use of such materials is quiteeconomical.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. In these examples, unless otherwiseindicated, parts and percent are given by weight and temperatures are indegrees Centigrade. It is to be appreciated, however, that theseexamples are merely exemplary of the present invention and are not to betaken as a limitation thereof.

EXAMPLE 1 An aqueous acid zinc plating bath was formulated containingthe following components in the amounts indicated:

Zinc sulfate l80 grams/liter Boric acid 23 grams/liter N-allylthiourea lgrams/liter One liter of this bath, at a pH of 4.5 and a temperature of24 C was air agitated and a bright zinc electroplate was obtained onsteel cathodes by electrolyzing the bath at a cathode current density ofabout 40 ampereslsquare foot, at plating times of from 20 to 40 minutes.

EXAMPLE 2 Into two (60) gallon tanks were added 74 lbs. of ZnSo, 9 lbs.of (Nl-l.,) SO and 10 lbs. or boric acid.

lnto tank A was added 21 grams thiourea while to tank B was added 3.5grams of N-allyl thiourea. The tanks were filled with water. Zinc wasplated for 400 amp hours/gal. of solution taking from two to threemonths to complete. During the electroplating, replenishment solutionwas added hourly. To tank A" was added 435 mg/hr. of thiourea while totank 8" was added 76 mg/hr. of N-allyl thiourea. At the end of theexperiment, the amount of thiourea used was calculated at 12.5 mg/amphour of plating time while N-allyl thiourea used only 2.09 mg/amp hourof plating time.

The results of the above described experiments clearly shown that anunsaturated thiourea, especially, N-allyl thiourea is surprisingly morestable than thiourea.

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention as changes therewithin arepossible and it is intended that each element recited in any of thefollowing claims is to be understood as referring to all equivalentelements for accomplishing substantially the same results insubstantially the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. An electroplating bath, which comprises an aqueous acidic solutionhaving present therein 50 to 400 grams of zinc sulfate per liter or 50to 300 grams of zinc fluoroborate per liter; 0.01 to 10 grams per literof a soluble thiourea brightening material selected from the groupconsisting of N-allylthiourea; N,N- ethylenethiourea;N,N'-dibutynylthiourea; and N,N' dibutenylthiourea; 10 to 30 grams perliter of an ammonium salt selected from the group consisting of ammoniumsulfate, ammonium chloride and ammonium bromide to improve theconductivity of the bath; 10 to 30 grams per liter of boric acid orammonium sulfate to maintain the pH of the bath during plating withinthe range of about 2.7 to 5.8; and l to 30 grams per liter of a materialselected from the group consisting of aluminum sulfate, dextrin,licorice and glucose to improve the crystalline structure of the zincplate obtained and to provide a wider operating range of currentdensity.

2. A method for the electrodeposition of a bright zinc plate whichcomprises electrodepositing zinc at an average cathode current densityof from about 10 to amps per square foot and the bath temperature withinthe range of about 15 to 40 C from the aqueous acid zinc electroplatingbath of claim 1.

2. A method for the electrodeposition of a bright zinc plate whichcomprises electrodepositing zinc at an average cathode current densityof from about 10 to 120 amps per square foot and the bath temperaturewithin the range of about 15 to 40* C from the aqueous acid zincelectroplating bath of claim 1.